This invention relates to a method and apparatus for controlling the idling rotation speed of an internal combustion engine driving a vehicle.
A closed-loop control method for controlling the idling rotation speed of an internal combustion engine driving a vehicle has been proposed in which, in order that the idling rotation speed of the engine can be controlled or maintained at a target or desired value, as designed, in a maintenance-free fashion, an analog computer or a digital computer is employed to calculate the deviation of the actual idling rotation speed of the engine from the desired speed, and then, a value of a control factor suitable for reducing the rotation speed deviation to a null is calculated, so that the amount of intake air or the amount of the air-fuel mixture supplied to the engine can be controlled according to the calculated value of the control factor.
In the prior art control method of the kind above described, however, no consideration is given to the limit of the control factor, and the closed-loop control is carried out only when the engine throttle valve is in its closed position. Thus, when the vehicle is being decelerated by depression of the foot brake pedal, and the actual rotation speed of the engine is lower than the desired rotation speed, the computer will calculate in such a way as to progressively increase the value of the control factor used for controlling the amount of intake air or the amount of the air-fuel mixture supplied to the engine, that is, in such a way as to progressively increase the rotation speed of the engine. Consequently, the rotation speed of the engine may become unusually high when the vehicle driver releases his foot from the foot brake pedal or disengages the clutch.
On the other hand, when the vehicle is running down a slope with engine braking or when the vehicle is in engine racing condition with the driver's foot being disengaged from the accelerator pedal, and the actual rotation speed of the engine is higher than the desired rotation speed, the computer will calculate in such a way as to progressively decrease the value of the control factor, that is, in such a way as to progressively decrease the rotation speed of the engine. Stalling of the engine will result when the vehicle driver disengages the clutch in such a state.
Thus, the computer controls the rotation speed of the engine against the will of the vehicle driver, and the vehicle driver will feel uneasy or unpleasant when the vehicle is running at a speed near the designed idling rotation speed of the engine.
The above problem can be solved by setting an upper limit and a lower limit of the control factor and limiting the control factor to within the range between the upper limit and the lower limit. However, such a manner of idling rotation speed control will become no more effective when dust and other foreign particles accumulate on the throttle valve or in a bypass passage bypassing the throttle valve due to a long period of time of use (aging) of the engine. This is because the value of the control factor used for the closed-loop control will become excessively large in such a situation until finally the relation between the control factor and its upper and lower limit will not be appropriate any more for the purpose of rotation speed control, and an engine trouble such as stalling of the engine will occur during a variation in the load or in a transient state of engine operation.